Process of producing by powder metallurgy techniques a ferritic hot forging of low flow stress

ABSTRACT

Ferritic age-hardenable alloy steels containing correlated percentages of nickel, copper, molybdenum, carbon, etc. in powder form are characterized by low flow stress, thus rendering them particularly suitable for P/M hot forging.

United States Patent Donachie [451 Feb. 11, 1975 1 PROCESS OF PRODUCING BY POWDER METALLURGY TECHNIQUES A FERRITIC I-IOT FORGING OF LOW FLOW STRESS [75] Inventor: Stephen James Donachie, New

Windsor, NY.

[73] Assignee: The International Nickel Company,

Inc., New York, NY.

[22] Filed: Mar. 29, 1973' [21] Appl. No.: 345,981

[56] References Cited UNITED STATES PATENTS 2,402,135 6/1946 Halley 75/125 X 3,720,512 3/1973 Yamaguchi et a1 75/226 X 3,795,129 3/1974 Goto 29/4205 X FOREIGN PATENTS OR APPLICATIONS 992,318 5/1965 Great Britain OTHER PUBLICATIONS L. Harrison et aL. Some Experiments in the Production of Low-Alloy Steel by Powder Metallurgy." Powder Metallurgy, 1962, No. 9, pp. 247-264.

Primary Examiner-C. W. Lanham Assistant Examiner-D. C. Reiley, lll

Attorney, Agent, or Firm-Ewan C. MacQueen; Raymond .1. Kenny [57] ABSTRACT Ferritic age-hardenable alloy steels containing correlated percentages of nickel, copper, molybdenum, carbon, etc. in powder form are characterized by low flow stress, thus rendering them particularly suitable for P/M hot forging.

3,132,025 5/1964 Hurley 3,303,061 2/1967 Wilson 75/125 X 5 Claims, 1 Drawing Figure DEQ EAS/NG ficw 67?:{85

l l l l l 1 I400 /500 /6oo 1700 1800 Ennsoerugrff) PROCESS OF PRODUCING BY POWDER METALLURGY TECHNIQUES A FERRITIC HOT FORGllNG OF LOW FLOW STRESS The subject invention is addressed principally to powder metallurgy hot forging of novel steels.

' As a review of the literature would confirm, powder metallurgy (often herein P/M) has continued to assume a more prominent position in many areas as a viable alternative to conventional melting-casting-working processing. This has been notably evident in respect of applications where the risks inherent in the uncertainties of segregation problems could not be entertained and, of course, in respect of those applications involving the production of intricately shaped components.

However, though the attributes of powder metallurgy are many, this unique tool has not escaped problemfree. As is known, powders are normally compacted and sintered in the conventional sequence of operations leading to a finished article. As a consequence, there is often encountered the attendant and inherent difficulties associated with porosity". Porosity simply means voids in the metal produced. As such, the voids can act in similar fashion to internal notches. By reason of this, inter'alia, conventionally produced P/M steels have hardly been known for their capacity to absorb much by way of impact energy, let alone significant levels thereof. Nor have they been known for an ability to resist fatigue stress. And it is axiomatic that porosity detracts from strength. Accordingly, the lack of such properties, toughness, fatigue life and yield and ultimate tensile strengths, has hampered the overall growth of PIM.

Techniques such as hot consolidation, repressing and/or infiltration, etc., have served to mitigate the porosity dilemma. But they are rather costly and usually not amenable to mass production, decided drawbacks. In recent years P/M hot forging has been resurrected, so to speak, since it offers a potential panacea to the porosity phenomenon while being responsive to automation. Here, however, initial die cost, short die life and operational down time for die replacement all combine to equal high die cost. Too, prior art steels have suffered from an inability to fill" complex dies requiring large amounts of metal flow, e.g., those used for automotive connecting rods.

Now, it appears to be generally acknowledged that lower forging pressures and/or temperatures would result in decreased die wear. In this connection, it has been reported that a decrease in forging pressure from 70 tsi to 30 tsi would improve dielife l 50p e rcent. Unfortunately, density drops 1.5 percent and at these levels impact resistance is related exponentially to density. The result enhanced die life at the drastic impairment of impact toughness, and fatigue resistance as well. This is most troublesome for most P/M hot forged parts envisaged to date would be exposed to cyclic or axial stress and impact loading, fatigue and toughness thus being of utmost importance. W NW 1 Hot Forging P/M Relationships Between Manufacturing Design, and Component Cost, T. W. Pietrocini, Society of Manufacturing E ngineers, Technical Pape r EN 71-260. 7

In any case, it has now been discovered that certain ferritic steel compositions containing correlated per-.

centages of nickel, copper, molybdenum, carbon, etc., can be forged at most dramatically reduced pressures and/or temperatures. It is considered that the flow stress of certain of such steels is so low as to be the virtual equivalent of pure iron at corresponding forging temperatures. These characteristics greatly promote improved die filling and bring about reduced die wear, lending to significant economic benefits.

Furthermore, in accordance herewith Charpy V- notch (CVN) impact values of up to ft. lbs. (room temperature) and fatigue limits of nearly 70 percent of the ultimate tensile strength have been achieved. These values obtain for steel compositions having yield and ultimate tensile strengths on the order of about 90,000l00,000 and 1 10,000] 50,000 psi, respectively, i.e., steels of intermediate strength. (The instant steels are not to be considered as high strength" steels, i.e., steels having yield strengths above about 125,000-150,000 psi.) Insofar as I am aware, typical impact and fatigue strengths of conventionally produced P/M steels forged at the same loads are generally on the order of about 5 to less than 25 ft. lbs. and 40 percent of ultimate tensile strength, respectively, at the comparable strength levels. It is considered that the high internal cleanliness of the microstructures of the instant steels, particularly a low oxygen content, lends to these qualities.

Generally speaking, the present invention contemplates the hot forging of age-hardenable ferritic steel powders which most advantageously contain about 0.7 to l or 1.25 percent nickel, about 1.4 to 2 or 2.25 percent copper, about 0.l5 to 0.35 percent molybdenum, up to 0.02 percent carbon, up to 0.05 or 0.1 percent silicon (if any), up to 0.15 percent manganese, the balance being essentially iron. Forging temperatures as low as 1300F. can be used, yet the densities of the forgings produced are at full density without recourse to excessive pressures.

ln carrying the invention into practice, a proper balance in chemistry must be struck to assure obtaining a iferritic (body-centered-cubic) structure. In addition, the alloying constituents should be correlated such that lthe Ac, critical temperature of the steels afford a substantially, if not completely, ferritic structure up to a jtemperature of 1400F. and most preferably to l550F., this to minimize the presence of the facecentered-cubic austenite. The presence of austenite is not only unnecessary but undesirable since it impedes flow stress. It should preferably not exceed 2 or 3 percent by volume, although higher percentages might be tolerated, say up to less than 10 percent or possibly in some instances up to 20 percent. Moreover, the alloying constituents should be balanced such that solid solution strengthening is maintained to a minimum during forging. Solid solution strengthening effects offset low flow stress. The foregoing alloying ranges are designed to achieve these characteristics. However, departures therefrom can be made using the following guides.

Nickel is an austenite former, lowers Ac contributes to aged tensile strength and impact energy and while it can be as high as 1.5 percent such higher percentages tend to unnecessarily decrease Ac, and this renders it more difficult to achieve the desired ferritic structure and lower flow stresses. The nickel level can extend down to 0.4 or 0.25 percent, but at the sacrifice of ,toughness and strength.

With regard to the element copper, it has but a moderate detracting influence with respect to the Ac, temperature. its main role is of imparting strength through precipitation hardening, although it does not appreciably contribute to solid solution strengthening during forging. The copper content can be as low as about 0.75 percent, but in striving for optimum results it should be at least 1.5 percent. Not much is gained by copper percentages above 2 or 2.25 percent. A range of 1.5 to 1.8 percent is very beneficial.

Molybdenum enhances the intensity of the copper age hardening reaction and raises the critical temperatures; however, it should not exceed 0.6 percent. High levels can introduce a solid solution strengthening problem during forging at low temperatures. I have found, for example, that an amount of molybdenum slightly above 1 percent did significantly increase tensile strength. But this solid solution hardening was achieved at the expense of flow stress and impact strength. And on balance the gain in strength neither warranted the increase in flow stress nor the loss of impact resistance. Molybdenum is also deemed to resist embrittlement. A range of 0.1 to 0.4 percent is satisfactory with a range of about 0.15 or 0.2 percent to 0.25 or 0.3 percent being considered the most advantageous.

The subject steels are of the low carbon type even to the point of being carbon-free. Carbon confers strength, but at the same time raises flow stress and it is deemed that the fatigue ratio (ratio of fatigue limit to tensile strength) is also needlessly decreased as well as the ability of the steels to absorb impact energy. For special purposes where relatively poor properties would be acceptable, carbon up to 0.1 percent might be tolerated in a carefully balanced alloy, but as a practical matter it should not exceed 0.03 to 0.05 percent. It is difficult to avoid the presence of carbon altogether, but notwithstanding this an upper level of 0.02 percent should be maintained.

As to the other constituents, the use of scrap stock in a melt charge would likely introduce manganese and also silicon. An increase in manganese results in higher levels of strength in the aged condition, but flow stress is also raised particularly if lesser amounts of other austenite stabilizing elements are not used. Moreover, manganese above 0.1 or 0.2 percent increases the oxide content and this should be avoided since oxides subvert toughness and fatigue characteristics. A manganese percentage of 0.5 percent can be tolerated where optimum results are not sought.

Silicon is a ferrite stabilizer and contributes to strength through solid solution hardening. It is a strong oxide former and detracts from toughness. Thus, it should be held to impurity levels, if any. Up to 0.3 percent can probably be tolerated where a lesser combination of properties can be accepted. Even here it should be held to less than 0.2 percent if at all possible.

Where scrap is used in a charge, up to 0.1% chromium could be present. However, while chromium is effective in raising the critical temperature of the steels contemplated, it is a very stable oxide former and for this reason it should be avoided. Where alloy cleanliness and other properties can be traded off for strength, then up to 0.3 or 0.5 percent can be tolerated.

Small amounts of boron can be employed, though it need not exceed 0.02 or 0.01 percent. Aluminum is unnecessary and should be controlled to a minimum, say 0.1 percent or lower. Phosphorus and sulfur should be held to not more than 0.04 percent, preferably to not more than 0.02 percent, each. Oxygen will be present and should be maintained, for reasons given above, to

not more than 0.06 percent, and most advantageously to not more than 0.02 percent.

Concerning the powder particles, while elemental powders might be blended and sintered to the desired composition, it is deemed preferable to use prealloyed powder. This can be accomplished through atomization in which a liquid melt is converted to powder by using air, inert gas, water, etc., to bring about atomization. Water atomization is considered appropriate since it is commonly employed, is relatively inexpensive, and provides particles of irregular shape. Prealloying and atomization also provide for small particle size and grain size. The alloy powders should not exceed about 500 or 600 microns (including oxide film), preferably being less than 250-300 microns.

The prealloyed powder particles are thereafter compacted to a preform, the shape of which will be often governed by the shape of the final product. Thereupon, the preform is heated to obtain the desired ferritic structure whereupon it is forged to shape and to full or nearly full density. As is rather conventional, an appropriate lubricant can be added to the powder before pressing to the preform. Also, the preform can, indeed should, be heated (sintered) prior to forging in accordance with usual practice. Subsequently, the product may, if desired and depending on composition, be further processed, e.g., machined, prior to aging. Steels in accordance herewith should be aged at about 900 to 1050F., e.g., 925 to 1000F., for about 1 to 5 hours. Above about 1000F. the alloys tend to overage, i.e., lose strength and gain in toughness.

In order to give those skilled in the art a better appreciation of the invention, the following is given.

Various steels, A, 1 and B in Table l, were prepared using electrolytic iron, nickel shot, ferromolybdenum percent Mo) and copper shot. The melt procedure involved forming an initial charge (45 kg) of iron, nickel and copper, heating to 3000F., adding the ferromolybdenum, and pouring at 3000F. into a heated tundish. The molten metal was water atomized at the bottom orifice of the tundish, the powder thereafter being dried and reduced at 1800F. (to obtain a good oxygen reduction) in a cracked ammonia atmosphere (dewpoint about minus 50F.). The powder was pulverized and heated for one hour at 1400F. (to remove strain from pulverization) under a cracked ammonia atmosphere.

The powders were admixed with a lubricant before compaction, in this case 0.5 percent by weight of Acrawax. The powders were blended with carbon, poured into a die and compressed cold. These green compacts were heated to 1200F. in cracked ammonia to dispel the Acrawax and cooled to ambient temperature. They were reheated to 2050F., again in cracked ammonia, and held thereat for one-half hour to effect sintering (approximately 6.79 gm/cm density).

To assess flow stress, a hot compression test was used. Specimens about 1/2 inch diameter and 1 inch in height were heated to 1450F. and a force was applied until the specimen height was reduced 0.2 inch, a 20 percent reduction.

The results are recorded in Table 1, crystal structure also being given. Included for comparison is a composition responding to A181 4620 (0.25 percent carbon added to the blend).

TABLE I Allo Ni Cu Mo C Mn Crystal Force at 20% No. Structure Compression lbs.

A151 4620 1.9 n.a 0.25 0.25" 0.1 l Austenite & 3400 Ferrite A 1.9 1.8 1.2 0.07 0.05 Ferrite 4050 1 0.95 1.9 0.3. 0.012 0.04 0.016 Ferrite 2550 B 0.95 1.9 0.3 0.1 0.04 Austenite & 3100 Ferrite added; n41. not added; Silicon 0.(ll in all alloys in respect ofthe above data, the compressive forming l5 46F2, 0.3 percent carbon added, was similarly proload required for MS] 4620 was relatively high, largely cessed as were (a) an A181 4600 type steel with a 0.3 by reason of excessive austenite. Alloy A shows that percent carbon addition, (b) AlSl 1050 (Fe 0.5 C) obtaining a ferrite structure per se is not necessarily an and Alloy A. answer. in this particular instance the high molybde- Against atheoretical maximum density of 7.84 -7.86, num content introduced excessive solid solution Alloy 1 had a density of 7.78 at l500F. versus only strengthening. This is in marked contrast with Alloy l, 7.55, 7.58, and 7.63 for the 46F2, 4600 and Alloy A an alloy within the invention. It will also be observed steels, respectively. A181 1050 manifested a density of that compressive forming for MS] 4620 was approxi- 7.69 which is higher than the other three, but it is a mately 33 /3 percent higher than for Alloy 1. This most steel very poor in terms of mechanical properties. It is significant advantage can be largely lost, for example, to be understood that these densities are not the best by the presence ofa comparatively high carbon level as that could be achieved. As will be appreciated by those evident from Alloy B. skilled in the art, the ton force (somewhat low) was Mechanical properties were determined in respect of deliberately selected so as to determine the ease by Alloys A and 1. In this connection, a preform specimen 30 which full density, if possible, could be reached. approximately 2 /2 X 0.4 X 0.75 inches was prepared Preform specimens of Alloy l and 46F2 and A181 and heated to forging temperature (1450F.) and 1050 steels were also forged over a range of temperapl ed i a confined, limiting flash die and forged at a ture to estimate expected flow stress behavior. The reforce of 70 tons. This produced a finished article apsults are depicted in FIG. 1 in which it can be seen that proximately 2-% X V2 X n inches. Both alloys were subthe expected flow stress of Alloy l is extremely low, opjected to tensile and impact testing with Alloy 1 also timum being at about 1450-l550F. It might be menundergoing an axial fatigue evaluation. Prior to test, the tioned that a flash die was used since the flash formed alloys were age hardened by heating to 950F. and is free to expand such that the lower flow stress materiholding for 4 hours. Alloy l was also tested after 5 815 Show a greater amount of flash, thus producing a hours at l00OF. The fatigue evaluation (Alloy l at maller forged height. 950F. for 4 hours) involved a more severe test than the Apart from the many advantages of the instant steels conventional tension-tension determination. In this re- 85 above dis ussed, it might also be mentioned that gard, complete reversal l di was l d howing to the unusually low flow stresses thereof, hot pull test) in which a cylindrical specimen was axially forgings have large surface areas or which require large loaded along its longitudinal axis. forging strains can now be forged at pressures capable TABLE II Alloy Y.S., UTS EL. RA. CVN* Fatigue Nu. psi psi ft. lbs. Limit, psi

70F; "heated-5 hr./l000F.

Concerning the mechanical properties reported in 60 of being delivered by available equipment. New capital Table II, particular note should be taken of the outinvestment will not be required. Simplified preform destanding impact and fatigue values. sign is another asset. For example, present internal in further confirming the effect of low stress characcombustion engine connection rod configurations are teristics on density characteristics of alloys within the so complicated that only a simple repressing can be acinvention, particularly the comparison with rather typicomplished. Furthermore, even conventional quenchcal current commercial P/M forgings, Alloys A and l were similarly processed as above except a forging force of 30 tons was employed. A steel designated ing and tempering is obviated and since minimum muchining is one of the principal economic advantages of hot forging-preforms, no machining is required to correct quenched induced distortion, an otherwise severe drawback. This provides for retention of closer part tolerances.

Moreover, the low oxygen content and clean structures greatly contribute to the overall combination of properties. Simply heating, e.g., sintering at 1900 to 2100F. in dissociated ammonia or equivalent is all that is required. Because the concentration of strong oxide formers is low in the subject steels, such a treatment results in very low oxygen contents, e.g., 0.01 or 0.02 percent and less. Of course, the oxygen content is low prior to the burn-off treatment, e.g, 0.2 percent or less due to initial low oxide content. Fatigue and impact resistance particularly benefit from such low oxygen clean forged structures. Such factors enable the steels to compete as a structural material at the given strength levels.

While the present invention is useful in the production of a wide variety of forged parts, it is deemed particularly applicable to the production of connecting rods, gearing, pinions and the like.

Although the invention has been described in conjunction with preferred embodiments, modifications can be resorted to. Apart from powder forging, the alloy powders can be extruded or otherwise worked. Compositions within the invention have been prepared by melting-casting-working procedures and exhibit useful properties for mill products though their structures are not as clean and they contain higher oxygen levels.

Such modifications are within the overall purview of the invention.

1 claim:

1. 1n the process of producing by powder metallurgy techniques a steel hot forging wherein die wear is reduced, which process comprises forming a preform from a ferritic alloy steel powder, and hot forging said preform at a temperature not greater than about 1550F., said powder consisting essentially of about 0.7 to about 1 percent nickel, 1.4 to 2 percent copper, 0. l 5 to 0.35 percent molybdenum, up to 0.02 percent carbon, up to 0.1 percent silicon, up to 0.15 percent manganese, the balance essentially iron, the forging being able to be carried out at such temperature largely by reason of the low flow stress characteristics of the steel composition.

2. A process in accordance with claim 1 in which the copper is from 1.5 to 1.8 percent and the molybdenum is from 0.2 to about 0.3 percent.

3. A process in accordance with claim 1 in which oxygen does not exceed 0.03 percent, silicon is less than 0.05 percent and manganese does not exceed 0.1 percent.

4. A process in accordance with claim 1 in which the hot forging temperature is from about 1400F. to less than about 1550F.

5. In the process of producing by powder metallurgy techniques a steel hot forging wherein die wear is reduced, which process comprises forming a preform from a ferritic alloy steel powder, and hot forging said preform at a temperature not greater than about 1550F., said powder consisting essentially of from 0.25 to less than 1.5 percent nickel, 0.75 to 2.25 percent copper, 0.1 to 0.6 percent molybdenum, up to 0.5 percent manganese, up to 0.3 percent silicon, up to 0.5 percent chromium, up to 0.02 percent boron, up to 0.05 percent carbon and the balance essentially iron. 

1. In the process of producing by powder metallurgy techniques a steel hot forging wherein die wear is reduced, which process comprises forming a preform from a ferritic alloy steel powder, and hot forging said preform at a temperature not greater than about 1550*F., said powder consisting essentially of about 0.7 to about 1 percent nickel, 1.4 to 2 percent copper, 0.15 to 0.35 percent molybdenum, up to 0.02 percent carbon, up to 0.1 percent silicon, up to 0.15 percent manganese, the balance essentially iron, the forging being able to be carried out at such temperature largely by reason of the low flow stress characteristics of the steel composition.
 2. A process in accordance with claim 1 in which the copper is from 1.5 to 1.8 percent and the molybdenum is from 0.2 to about 0.3 percent.
 3. A process in accordance with claim 1 in which oxygen does not exceed 0.03 percent, silicon is less than 0.05 percent and manganese does not exceed 0.1 percent.
 4. A process in accordance with claim 1 in which the hot forging temperature is from about 1400*F. to less than about 1550*F.
 5. IN THE PROCESS OF PRODUCING BY POWDER METALLURGY TECHNIQUES A STEEL HOT FORGING WHEREIN DIE WEAR IS REDUCED, WHICH PROCESS COMPRISES FORMING A PREFORM FROM A FERRITIC ALLOY STEEL POWDER, AND HOT FORGING SAID PREFORM AT A TEMPERATURE NOT GREATER THAN ABOUT 1550*F., SAID POWDER CONSISTING ESSENTIALLY OF FROM 0.25 TO LESS THAN 1.5 PERCENT NICKEL, 0.75 TO 2.25 PERCENT COPPER, 0.1 TO 0.6 PERCENT MOLYBDENUM, UP TO 0.5 PERCENT MAANGANESE, UP TO 0.3 PERCENT SILICON, UP TO 0.5 PERCENT CHROMIUM, UP TO 0.02 PERCENT BORON, UP TO 0.05 PERCENT CARBON AND THE BALANCE ESSENTIALLY IRON. 